1. Field of the Invention
The present invention relates to automatic transmissions and gear trains. In particular, the instant invention relates to an automatic transmission and gear train with improved resistance to deterioration with an increased effective life.
2. Description of Related Art
Motorized vehicles include a powertrain that is comprised of an engine, multi-speed transmission, and a differential or final drive. The multi-speed transmission increases the overall operating range of the vehicle by permitting the engine to operate through its torque range a number of times. The number of forward speed ratios that are available in the transmission determines the number of times the engine torque range can be repeated. Early automatic transmissions had two speed ranges. This severely limited the overall speed range of the vehicle and therefore required a relatively large engine that could produce a wide speed and torque range. This resulted in increased engine wear and tear and reduced engine efficiency. As a result, manually-shifted transmissions that offered several speed ratios were the most popular for use in motor vehicles.
With the advent of three- and four-speed automatic transmissions, the automatic shifting (planetary gear) transmission increased in popularity. These transmissions improved the operating performance and fuel economy of the vehicle. The increased number of speed ratios reduces the step size between ratios and therefore improves the shift quality of the transmission by making the ratio interchanges substantially imperceptible to the operator under normal vehicle acceleration.
One aspect of the instant invention is a redesign of an automatic transmission that has a high failure rate in its present application. The U.S. Postal Service (USPS) fleet has 160,000 Long Life Vehicles (LLV's) on the road with a service life of 25 or more years. These vehicles rely on the 180C and/or the 700R4 transmission. There has been a long felt need to design a total transmission system specifically for the vehicle driving conditions of this fleet that would provide durability, longevity and cost savings in the field. High vehicle down time due to inherent original transmission design problems have been plaguing this fleet for many years. During this time, the USPS has utilized local and regional rebuilders to supply replacement products to keep their fleet on the road at great cost.
Due to a shortage of quality replacement parts for the 180C transmission, a 700R4 transmission was introduced in the late '90's to replace the 180C. The 700R4 transmission is a GM unit that has been in the marketplace for many years. This unit was not designed for the Postal LLV application and had to be modified to adapt to the vehicle. This kit included a driveshaft, shift linkage and torque converter. The lack of controlled testing and evaluation of the 700R4 transmission resulted high failure rates in the field. Thus, there is a clear need for a transmission that is an improvement over the 180C and 700R4 transmissions.
The 180C transmission has problems in five areas:    1. Planetary gear wear or tooth breakage.    2. Valve body malfunction. (2–3 shift valve sleeve sticks)    3. Torque converter fails.    4. Main pump fails.    5. Low band fails.
The nature of the use of the 180C transmission by the Postal Service results in constant shifting from first gear to second gear and back down to first gear. This shifting sequence occurs hundreds of times per vehicle in one day of service. The 180C transmission was designed to shift first to second and then to third. The converter clutch would then be applied and the driver and vehicle would proceed to its destination. Thus, most of the transmission's useful service would be spent in third gear. The 180C transmission as used by the Postal Service, and in other applications which require a great deal of stop and go driving, does not see such ideal usage. After repeated first to second and then second to first shifts the pinion gears within the 180C gear train begin to spall. The spalling of the gears results in very small yet very hard pieces of steel flaking off the pressure angle surface of each pinion gear. These pieces of steel are washed away from the pinion gears by the oil that lubricates and cools these same gears; thus the pieces of hard steel are suspended in the oil. As the contaminated oil is pumped to all critical areas of the transmission, these pieces of metal act as an abrasive to all parts they contact. The abrasive action of the steel particles wears the surfaces of all mating and moving parts. As gear deterioration progresses the hard steel particles become more concentrated in the oil and become lodged in the valves of the valve body. This results in the transmission not shifting back to first gear, but instead using second as the lowest gear. This phenomenon was confirmed in transmission destructive tests. In order to continue the test, the valve body had to be replaced for the transmission to operate in a normal manner.
Other affected parts are the torque converter and the main pump assembly. It is difficult to determine which of these assemblies fail first since they are connected and the contaminated oil is pumped from one to the other. Observed deterioration includes the converter hub wearing where it is supported by the pump bushing, converter internal bearing deterioration, pump outer and inner gear wear, and pressure regulator valve malfunction. All of these malfunctions are the direct result of the suspended metal in the oil.
This deterioration can lead to main pump malfunction and oil loss between the converter hub and the pump. The loss of shifting control and pump pressure results in the rise of transmission temperature, with temperatures of three hundred degrees or more conceivable. Under these conditions the seals that hold the oil pressure that applies the clutches and bands begin to harden and allow for oil leakage, thus also allowing the clutches and band to slip under load, which leads eventually to clutch and band failure. At this point complete transmission failure occurs. The vehicle will not move.
The conventional planetary gear system in the 180C transmission includes a sun gear and a ring gear with pinion gears engaged between the sun and ring gears. The pinion gears are connected to and carried by a pinion carrier assembly. The pinion carrier assembly includes a pinion carrier body and pinion shafts, the pinion shafts being fixed to the pinion carrier body. The 180C utilizes a three-pinion arrangement of compound and plain gears (six gears total, three compound and three plain). As stated above, this transmission fails because of the nature of the application. The repeated shifts between first and second gear causes spalling of the pinion gears. The solution to the above described problems is to prevent this spalling. The instant invention comprises a system that would substantially extend the life of the average 180C transmission. This newly rebuilt transmission is the 280PS™ transmission.